Torpedo body form and gas layer control



T. G. LANG Sept. 14, 1965 TORPEDO BODY FORM AND GAS LAYER CONTROL 2 Sheets-Sheet 1 Filed Jan. 7, 1964 INVENTOR.

THOMAS G. LANG wnw ATTORN EY Sept. 14, 1965 T. e. LANG TORPEDO BODY FORM AND GAS LAYER CONTROL Filed Jan. 7, 1964 2 Sheets-Sheet 2 STABILIZATION THROTTLE S S' CIRCUITS VALVE CAVITY INVENTOR.

THOMAS 0. LANG 9 3.W' ATTORNEY.

3,205,846 TORPEDO BODY FQRM AND GAS LAYER CONTROL Thomas G. Lang, South Pasadena, Calif, assignor to the United States of America as represented by the Secretary of the Navy Filed Jan. 7, 1964, 'Ser. No. 336,328 7 Claims.- (Cl. 114-67) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to underwater vehicles, such as torpedoes, and more particularly to apparatus for introducing and maintaining a gaseous medium along a portion of the outer surface of the vehicle in order to reduce drag.

Recent interest has been shown in introducing a gaseous medium along the surface of a torpedo to reduce wetted frictional drag. The impetus for this interest is, in part, due to the use of newer combustion type propulsion units employing gaseous products derived from a combustive process, such as the burning of a stoichiometric solid propellant or the combustion of slurried mixtures. The gaseous products are employed as the working fluids to operate some form of engine for producing shaft rotation, and after the gases have performed the working portion of their cycle they must be exhaustedf-rom the torpedo. This exhaust provides a source of pressurized gases for drag reduction apparatus, which is conveniently available without added weight or volume penalty. As far as known, heretofore all efforts to reduce drag by gas introduction have not met with substantial success or come into practical use. One of the major problems in the use of a gas along the surface of an underwater vehicle is maintaining the gas in stable dynamic equilibrium under the pressures imparted to it by turbulent conditions of the ambient stream of sea water, and especially in connection with pressure changes caused by maneuvering of the torpedo.

An object of the invention is to provide novel apparatus for introducing and maintaining a gaseous medium along the outer surface of an underwater vehicle which significantly reduces drag.

Another object is to provide apparatus in accordance with the preceding object in which the gas is maintained in stable dynamic equilibrium under the turbulence which may be encountered in sea water under maneuvering of the vehicle.

A further object is to provide a novel torpedo employing the gaseous exhaust of a combustion type propulsion unit to reduce drag.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the inveniton when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a side elevation, partially in section, of a torpedo in which a gaseous medium is introduced along the hull in the form of a gas cavity, with certain features shown schematically and to an exaggerated scale;

FIG. 2 diagrammatically illustrates the variation of length and convergency of the cavity in FIG. 1, under variations in rate of flow of gas into the cavity;

FIG. 3 is a detail taken at arrow 3, FIG. 1;

FIG. 4 is an electrical circuit diagram;

FIG. 5 illustrates a modified form of invention; and

FIG. 6 illustrates another modified form of invention.

Referring now to the drawing and in particular to FIG.

\ United States Patent 0 3,205,846 Patented Sept. 14, 1965 1, a torpedo 10 has a hull having a longitudinal hydro dynamic axis B parallel to the direction of torpedo movement. Briefly, and as will hereinafter be amplified upon, the hull at its front end comprises a nose section 12 forming a flat surface 14. Surface 14 is inclined downward in the forward direction, by a predetermined angle X between axis B and a reference axis C normal to surface 14. Gases are exhausted from the torpedo directly behind nose portion 12, and under forward movement of the torpedo, these gases form an annular cross sectioned rearwardly extending gas cavity 15 having a gas-to-waterinterface 16 of a predetermined shape determined by the rate of flow of gas into the cavity and by hydrodynamic and gravity or buoyancy forces. A mid-section 17, which constitutes the major portion of the length of the torpedo, is disposed within cavity 15. The outer surface of midsection 17 is designed to be in a predetermined space relationship from cavity interface 16. Extending rearwa-rdly from midsection 17 of the hull is a tail section 20 having a surface of predetermined shape. Adjacent to mid-section 17 the surface of tail section 20 forms an outwardly convex surface of revolution 22 about an axis of symmetry E, which convex surface is initially divergent and then convergent at it extends in the rearward direction. Joining and merging with the rear end of surface 22 is an inwardly concave surface of revolution 24 converging in the rearward direction and terminating at its rear end in a curve substantially forming a tangent to the cylindrical surface of the propeller shafting. Axis of symmetry E, is angularly disposed relative to hydrodynamic axis B by a predetermined negative (in the rearward direction) angle Y. Aflixed to tail section 20 is a shroud ring 26 which converges in the rearward direction, having an inwardly convex inner surface 27a and an inwardly concave outer surface 27b. Forward propulsion is provided by an engine 28 of the type in which gaseous products are derived from a combustion process, and a portion of the exhaust formed by these gases constitutes the supply of gases to be delivered into cavity 15. Engine 28 conventionally drives a pair of contra-rotating propellers 32 which have hollow hubs forming one of the channels for venting the engines exhaust gases. A steering control is provided, as by gas outlet ports (not shown) along the surface of shroud ring 26, such as is described in U.S. Patent 3,096,739 to K. E. Smith, entitled Method and Apparatus for Steerin g Underwater Bodies.

Another exhaust line 34 from engine 28 forms the supply line for the cavity gases. Operatively connected in gas line 34 is an electrically controlled throttle valve 35 which is controlled by a feedback type control circuit 36 which is Operatively responsive to a pivoted paddle-like sensing device 36a, best shown in FIG. 3, for measuring the distance between surface 18 of midsection 17 and the cavity interface 16. The outlet side of valve 35 delivers a variable flow rate of exhaust gas to one branch of a Y connection 37, and thence to the inlet of a fixed speed motor driven pump 33. The output of pump 38 is connected to a pump delivery line 40 which communicates the gases to the center of a generally transverse outlet conduit 44. This conduit is formed by the mounting nose portion 12 in spaced relationship to midsection 17, by means of suitable struts 45 and thereby forming a circumferential gas outlet 46 opening to the hull of the torpedo at the junction between nose portion 12 and midsection 17. The circumferential marginal portion of frontal surface 14 is faired in a manner to form a sharp circumferential edge 48. The forward end of outer surface 18 of mid-section 17 is recessed relative to edge 48 so that edge 48 forms a sharp discontinuity between frontal surface 14 and the forward end of surface 18. Cavity 15 is formed when gases are exhausted from conduit 44 at a sufficient rate of flow to maintain the gas pressure in the cavity at a level slightly under the static pressure of the stream of ambient water. The sharp discontinuity provided by edge 46 aids in the formation of a well defined interface 16, which when well formed appears as a glass wall with virtually no disturbance. A tubular ring 51, having an annular opening 52 at its rear edge, is disposed between the rear edge of mid-section 17 and convex surface 22 of tail section 20, the annular opening communicating with an annular gas intake 54. A line 56 communicates the gases received by ring 51 with the other branch of Y junction 37 and thence to pump 38. Pump 38, gas outlet 46, gas intake 54, and their connecting lines form a pumping system which sucks the gases not entrained in the ambient water stream into intake 54 for re-circulation.

Referring now to FIG. 2, it has been found that there is a predetermined relationship between the pressure maintained in cavity 15 under variations in the flow rate of gases through outlet 46, and the shape and size of cavity interface 16 relative to the body of torpedo 10. Increasing the cavity pressure causes cavity interface 16 to diverge outwardly, and extends the length of the cavity, with the incremental extension of length for a given increase in pressure many times greater than the incremental increase in divergency. Cavity interface shape and size for three discrete cavity pressures of progressively increasing value are shown diagrammatically as interface contour lines 16a, 16b, 16c having cavity closure zones 58a, 58b and 580, respectively. Curve 16a terminates somewhat ahead of convex surface 22; curve 16!) is more divergent than curve 16a and terminates by tangential intersections of the interface with the hull in a region extending rearward from a forward limit defined by the widest part of convex surface 22 and including the region of transition between convex surface 22 and concave surface 24; and curve 160 is more divergent than curve 16b, terminating near the rear end of concave surface 24. In the case of interfaces 16a and 160 where the interface does not tangentially intersect the hull, there is a splashing against the hull, schematically representing the interface terminating in a sharp inwardly directed curve, due to the natural tendency of a cavity to form a re-entrant jet where it terminates. As will become apparent, the tangentially intersecting contour line 16b is the predetermined shape achieved in the present invention.

The method and means by which the aforementioned predetermined relationship between cavity pressure and shape and size of cavity interface are employed to maintain a predetermined shape of interface 16 will now be described. Paddle device 36a for measuring distance G is mounted on the hull of the torpedo somewhat ahead of the junction of mid-section 16 and tail section 20. Paddle device 36a comprises an elongated element pivoted between its ends about a pivot axis H. One of the pivot arms of the element forms a vane or paddle arm 66 which extends outwardly from the torpedo hull. The other pivot arm forms a voltage pick-off wiper 68 which wipes across a feedback potentiometer winding 70. Element 64 is resiliently biased about axis H toward an upright position defined by a limit stop 72 by a tension spring 74. The length of paddle arm 66 is so chosen that a part of its paddle area will be disposed in the ambient stream beyond cavity interface and a part disposed in cavity 49. A positive DC. voltage +V is applied to the end of potentiometer winding 70 remote from the limit stop end. The ambient stream produces a force upon paddle arm 66 acting in a direction opposite to the force of spring 74, and with a force proportional to the proportion of the paddle area immersed in the stream. As distance G increases, less of paddle arm 66 is immersed in the stream, with the result that the 4 force acting upon the paddle arm, and in turn the voltage at pick-off wiper 68 are decreased. Thus the voltage at pick-off wiper 68 varies in accordance with an inverse relationship to distance G.

The voltage at pick-off wiper 68 is applied as the input feedback circuit 36, best shown in FIG. 4, which comprises a presettable cavity length potentiometer 76 having an adjustable pickoif contact 78 which may be selectively pre-positioned along a potentiometer winding 80 by a suitable setting knob 82. A DC. voltage, -V, of opposite polarity to that across the potentiometer winding 70, is applied across Winding 80. The setting of arm '78 requires the determination of the exact dimensions of a cavity that will have an interface that essentially follows the contour line 16b, FIG. 2, that is to say a cavity of a predetermined shape such that the natural tendency to collapse occurs somewhere within the aforementioned limits. In the conventional manner of closedloop control circuits, the voltage at adjustable pickotf contact 78 is an electrical signal order to control throttle valve 35 to maintain the distance G at a value determined by the setting of knob 82, corresponding to that of the desired cavity shape. The output of summing network 84 is a D.C. voltage having a polarity and magnitude corresponding to the sign and magnitude of the algebraic sum of its D.C. input voltages. Since the potential applied to potentiometer windings 7t and 30 ar of opposite polarity, the output of summing network 84 is of one or the other polarity depending upon whether the actual distance G is less than or greater than the distance called for by setting of adjustable contact 78, and has a magnitude in accordance with magnitude of difference between such distances. The output from summer 84 is applied through suitable amplifiers and stabilization circuitry 86 for providing smoothness of operation to the control input of throttle valve 35, which is adapted to selectively increase the rate of flow of the pump when distance G is less than the desired distance, and to decrease the rate of flow when greater than the desired distance. This in turn increases or decreases the divergency of the cavity until the actual distance G matches the desired preset distance. The action of closed loop circuit 36 is, of course, continuous and practically instantaneous and maintains distance G substantially constant. The response of the cavity has been symbolically shown as a block 88 which closes the circuit loop. In summary, the primary purpose of sensor 36:: and circuit 36 is to maintain the shape and size of cavity interface 16 essentially as represented by contour line 16b, FIG. 2, in order to communicate the cavity to a predetermined circumferential locus of cavity closure, which may be otherwise considered the locus at which the torpedo is rewetted or the locus of reattachment of the cavity Wall to the torpedo hull. is located between a forward limit defined by the widest portion of convex surface 22 and a region extending rearwardly from such region including the region of transition between convex surface 22 and concave surface 24.

As previously mentioned, hull 18 is in predetermined spaced relationship from the predeterminedly shaped cavity interface 16, and it will now be apparent that the predetermined cavity interface is the one produced under control of the feedback control circuit 36 in response to its distance-to-interface measuring device 36a. In order to provide the desired spaced-apart-relationship the shape of the cavity interface must be first determined, and this is most conveniently done by calculations through analogy to water tunnel investigations of a cavity formed by a fiat disk at an angle of attack in a moving fluid. The use of water tunnel investigations and conventional techniques to determine the cavity shape produced by a disk under angle of attack is thoroughly discussed and described in Cavity Shapes for Circular Disks at Angles of Attack, 21 report by R. L. Waid, published 1959, the

This predetermined locus California Institute of Technology, to which reference is made. As will be apparent from the discussion in this publication, since cavity is formed behind the down- Wardly-inclined surface 14, its shape is such that the center line K of the cavity slopes negatively (in the rearward direction) at the torpedo nose, With the slope of the centerline K gradually zero at about the middle of the torpedo, and with the centerline K having a gradually increasing positive slope in the rear half of the torpedo. The initial negative slope is the result of the downward forces exerted on the stream by the inclined frontal surface 14, and the change to a zero slope and then an increasing positive slope is the result of the cumulative upward displacement of the gas cavity by buoyancy forces. It is to be appreciated that since centerline K is a downwardly convex curve, the upper contour line 16,U of the cavity interface has somewhat less curvature than the lower contour line 16L. The cross sections throughout the length of the cavity is essentially circular.

Once the shape of interface 16 is determined, surface 18 is spaced radially inwardly therefrom by a distance sufficient to allow spacing between the hull and cavity interface to be greater than the thickness of the boundary layer of gas along surface 18 at all points along the length of the cavity. The thickness of the gas boundary layer along the torpedo may be determined by conventional techniques. As the result of this spacing, the

gas velocity is essentially uniform along the gaswater interface. A preferred selection of spacing in conjunction with the selection of other operating parameters of gas circulation through the cavity, is one in which the gases flow along the length of the cavity with a velocity essentially equal to the velocity of the ambient stream, thereby eliminating slippage between the gas and water fluid mediums at interface 16 and inhibiting any wave formation at the interface. This increases the stability of interface 16, which in turn improves the stability of control action in response to distance-to-interface measuring device 36a. Elimination of slippage also results in an added margin of drag reduction through reduction of friction at the interface.

As mentioned, the cavity interface 50 is controlled to terminate along a predetermined portion of tail section 20, and conversely the shape of tail section 20 is calculated and selected to match the shape of the cavity interface for the desired length of cavity, the two being interdependent. It is to be appreciated that a purpose of the outwardly convex surface 22 is to provide a surface that planes against the cavity interface 16. Such planing action provides a stabilizing influence upon the closure of the cavity, and in particular counteracts a tendency to produce spray at the zone of cavity closure, which has been observed to exist even in the case of ideal tangential intersection of interface and hull and is probably due to a tendency to form a reentrant jet. In a preferred condition of adjustment of the shape of interface 16 and tail section 20, the intersection of the tail section surface and the interface occurs at the region of transition between convex surface 22 and concave surface 24.

The location of gas intake 54 facilitates withdrawal of gases from the cavity closure zone, and therefore tends to inhibit entrainment of cavity gases in the ambient stream, which in turn reduces drag losses due to gas entrainment. Also, any factor exerting a stabilizing influence in the region of .cavity closure tends to reduce the size of bubbles of cavity gas that are entrained which further reduces drag losses.

In accordance with well known conventional theory of hydrodynamic design, the drag losses due to the separation of the flow by the torpedo nose, commonly thought of as the loss due to pushing the torpedo through water, may be quite substantially compensated for by a hydrodynamic thrust upon the tail of the torpedo produced by rerouting of the flow stream in a manner providing rearward and inward acceleration of the flow in the region of the tail. The hydrodynamic thrust referred to is entirely apart from the thrust produced by the propulsion plant, and may be thought of as neutralization of the nose drag by a recovery of losses through the hydrodynamic shape of the torpedo. Concave surface 24, which is sometimes referred to as a cusp-shaped surface serves to thusly re-route the flow in the desired manner. Also, the location of shroud ring 26 and the shape of its surfaces 27a and 27b are selected to produce the desired re-routing flow. In rerouting flow, concave surface 24 and shroud ring 26 both further serve to exert a stabilizing force on the flow in the region of cavity closure. Conversely it is essential to production of forward thrust that the tail of the torpedo be fully wetted, and therefore any factor that exerts a stabilizing effect upon cavity closure and therefore provides smoother re-wetting of the tail serves to augment the hydrodynamic thrust exerted on the rear end of the torpedo. The static pressure within cavity 15 is uniform throughout its volume as the result of the previously described choice of gas layer thickness. Therefore, the pressure acting on the top and bottom of midsection 17 is the same. This is in contrast to the case of a fully wetted torpedo in which a difference between static pressures at top and bottom of displaced volume of water produces a buoyancy force which acts on the torpedo in a manner tending to overcome its weight. A conventional fully wetted torpedo is normally adapted to employ these forces of buoyancy to neutralize its weight and thereby allow it to fly through water rather than drop under its own weight. However, since the forces of buoyancy do not act along mid-section 17 of torpedo 10, the torpedos weight must be otherwise neutralized. Stated another way, the absence of buoyency must be compensated. It will now be apparent that the purpose of forming frontal surface 14 with its normal axis C angularly disposed to hydrodynamic axis B, is to provide a lifting surface at the nose to compensate for such absence of buoyancy, and angle X is calculated and selected to provide one-half the required lifting forces, a typical value being in the order of a few degrees. Similarly, the purpose of forming tail 20 with its axis of symmetry E angularly disposed to axis B is to provide a lifting surface at the tail, and angle Y is calculated and selected to provide one half the required lifting forces, a typical value also being a few degrees.

FIG. 5 shows an alternative arrangement at the junction between mid-section 17' and a tail section 94, which may be used in conjunction with the described method of generation of an annular cavity and controlling its length. Tail section 94 carries a plurality of propeller blades 96 and is rectilinearly moveably mounted to the mid-section 17' of the hull by means of a splined drive shaft 98. A compression spring 99 resiliently biases the tail section 94 toward a rearward limit stop position defined by engagement of an abutment 100 carried by shaft 98 against the front end of a groove 102 in which the abutment rides. Tail section 94 has a rearwardly tapering conical surface 104 with a cylindrical surface 106 joining'the conical surface at its front end. The geometry of cylindrical surface 106 is so chosen that it will extend ahead of cavity interface 16' when tail section 94 is in its most rearward limit stop position. Under forward movement through the water, the previously described forwardly directed hydrodynamic forces associated with the re-routed flow along conical surface 104 acts against the bias of spring 99, moving tail section 94 forward to a position at which the forward thrust and the resilient force of spring 99 are in equilibrium. This equilibrium position of tail section 94 inherently corresponds to a position providing stable closure of the cavity 15'. This construction and arrangement achieves rewetting and forward hydrodynamic thrust, but does not require a gas intake. Cavity gases become entrained in the how stream passing along surface 104. Steering control is conventionally provided by steering fins 105 mounted to mid-section 17.

FIG. 6 is another alternative arrangement for use with a cavity of controlled length. Provided ahead of the conical tail section 118 is a water recirculation channel 108 having an intake 110 formed in the leading edges of the fins 112. Pump blades 113 are provided in channel 108 to produce a circulating flow of water along the doughnut-shaped channel. The front outer edge 116 of tail section 114 is formed as a sharp edge and is recessively stepped from the outer interface 117 of the recirculating flow to cause a portion of the recirculating flow to move along the conical surface 118 of the tail section, and thereby direct a sheet-like jet of water rearwardly along conical surface 118. The outer interface of the rearwardly directed jet substantially tangentially merges with interface 16" of the cavity with the result that termination of cavity 15 is effected by the merging two water walls. The rearwardly directed jet also provides propulsive thrust. Again gas withdrawal is not needed.

In summary, it will be apparent that a torpedo has been described in which only a small portion of the nose and tail sections are in direct contact with the ambient water and wherein the large majority of the torpedo length is effectively immersed and traveling through a substantially inviscate gaseous medium. Accordingly, the frictional resistance or drag upon surfaces of the mid-section is essentially identical to that which would result if traveling through gas rather than water, with resultant overall reduction of drag of a torpedo. This construction and arrangement reduces the skin friction resistance of torpedoes over that presently existing by as much as a factor of four or five. Since both the nose section and the tail section are fully wetted and the tail section is designed to smoothly close the cavity and re-route the flow in a manner providing forward hydrodynamic thrust, the separation drag losses produced at the nose are to a large degree recovered.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An underwater craft comprising:

(a) an elongated hull having generally rounded transverse sections therealong,

(b) means for generating an annular gas cavity adjacent to the hull and for communicating the cavity rearwardly from a predetermined circumferential cavity generation locus of the hull disposed near the nose of the craft to a predetermined circumferential cavity closure and rewet locus of the hull disposed near the tail of the craft, whereby skin friction of the hull between the loci is substantially reduced,

() said means for generating and communicating the cavity including means for introducing a gas into the cavity and for selectively varying the quantity of gas introduced into the cavity to maintain same communicated between the loci,

(d) means for measuring the thickness of said annular cavity, said means adapted to introduce a variable quantity of gas into the cavity, and

(e) including means responsive to said thickness adapted to control the quantity of gas introduced into said cavity in an inverse relationship to the cavity thickness.

2. An underwater craft in accordance with claim 1,

(f) said thickness of the annular cavity being measured at a station near the rewet locus.

3. An underwater craft comprising:

(a) an elongated hull having generally rounded transverse sections therealong,

(b) means for generating an annular gas cavity adjacent to the hull and for communicating the cavity rearwardly from a predetermined circumferential cavity generation loci of the hull disposed near the nose of the craft to a predetermined circumferential cavity closure loci disposed near the tail of the craft, whereby skin friction of the hull between the loci is substantially reduced,

(0) said means for generating and communicating the cavity including means for introducing gas into the cavity,

(d) said hull being radially inwardly spaced from said gas-water interface by a distance greater than the thickness of the boundary layer of gas adjacent to the hull, and

(e) means for generating upward lift to compensate for absence of buoyant forces between said loci,

(f) the portion of the hull ahead of the" cavity generation locus being so constructed to produce approximately one-half of said upward lift, and

(g) the portion of the hull behind the closure locus being so constructed to produce approximately onehalf of said upward lift.

4. An underwater craft, comprising;

(a) an elongated hull having generally rounded transerse sections therealong,

(b) means for generating an annular gas cavity adjacent the hull and for communicating the cavity rearwardly from predetermined circumferential cavity generation locus of said hull near the nose of the craft to a predetermined circumferential cavity closure locus of the hull disposed near the tail of the craft, whereby skin friction of the hull between the loci is substantially reduced,

(0) said hull and said means for generating and communicating a cavity adapted to cooperate to rewet the hull aft of the cavity closure locus, and

(d) a tail section adjacent the cavity closure locus forming an outwardly convex surface of revolution which in a rearward direction initially diverges and then converges,

(e) said convex surface adapted to tangentially intersect the gas-to-water interface of the cavity.

5. An underwater craft in accordance with claim 4,

including,

(f) means for withdrawing gas from the cavity from the zone adjacent the convex surface ahead of its intersection with the gas-to-water interface.

6. An underwater craft in accordance with claim 5,

including (g) a shroud ring disposed about said tail section,

(b) said shroud ring being inwardly cambered and having a leading edge diameter larger than the trailing edge diameter.

7. An underwater craft comprising:

(a) an elongated hull having generally rounded transverse sections therealong,

(b) a circumferential gas outlet near the front end of the craft, and

(c) an associated gas supply connected thereto,

(d) said outlet and said gas supply adapted to generate an annular gas cavity adjacent the hull extending rearwardly from said outlet along the hull under forward movement of the craft,

(e) means for measuring the distance between the hull and the gas-to-water interface of the cavity,

(f) means for varying the rate of flow of gas through said outlet responsive to the distance between the hull and the outer gas-to-water interface of the cavity,

(g) said hull and said means for varying the flow of gas adapted to cooperate to close the cavity at a predetermined circumferential locus near the end of the craft,

brium of gas within the cavity by forces exerted 5 thereupon by the ambient stream, and

(i) means to provide upward lift to the hull to compensate for the absence of buoyancy effects along the portion of the hull surrounded by the cavity.

References Cited by the Examiner UNITED STATES PATENTS 3,040,688 6/62 Gram 11467 3,075,489 1/63 Eichenberger 114-67 3,104,641 9/63 Froehlich 114-16 MILTON BUCHLER, Primary Examiner.

FERGUS S. MIDDIJETON, Examiner. 

1. AN UNDERWATER CRAFT COMPRISING: (A) AN ELONGATED HULL HAVING GENERALLY ROUNDED TRANSVERSE SECTIONS THEREALONG, (B) MEANS FOR GENERATING AN ANNULAR GAS CAVITY ADJACENT TO THE HULL AND FOR COMMUNICATING THE CAVITY REARWARDLY FROM A PREDETERMINED CIRCUMFERENTIAL CAVITY GENERATION LOCUS OF THE HULL DISPOSED NEAR THE NOSE OF THE CRAFT TO A PREDETERMINED CIRCUMFERENTIAL CAVITY CLOSURE AND REWET LOCUS OF THE HULL DISPOSED NEAR THE TAIL OF THE CRAFT, WHEREBY SKIN FRICTION OF THE HULL BETWEEN THE LOCI IS SUBSTANTIALLY REDUCED, (C) SAID MEANS FOR GENERATING AND COMMUNICATING THE CAVITY INCLUDING MEANS FOR INTRODUCING A GAS UNTO THE CAVITY AND FOR SELECTIVELY VARYING THE QUANTITY OF GAS INTRODUCED INTO THE CAVITY TO MAINTAIN SAME COMMUNTICATED BETWEEN THE LOCI, (D) MEANS FOR MEASURING THE THICKNESS OF SAID ANNULAR CAVITY, SAID MEANS ADAPTED TO INTRODUCE A VARIBLE QUANTITY OF GAS INTO THE CAVITY, AND (E) INCLUDING MEANS RESPONSIVE TO SAID THICKNESS ADAPTED TO CONTROL THE QUANTITY OF GAS INTRODUCED INTO SAID CAVITY IN AN INVERSE RELATIONSHIP TO THE CAVITY THICKNESS. 